Carbon is one of the most important known elements and can be combined with oxygen, hydrogen, nitrogen and the like. Carbon has four known unique crystalline structures including diamond, graphite, fullerene and carbon nanotubes. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall or multi-wall, and commonly referred to as single-walled nanotubes (SWNTs), or multi-walled nanotubes (MWNTs), respectively. These types of structures are obtained by rolling a sheet formed of a plurality of hexagons. The sheet is formed by combining each carbon atom thereof with three neighboring carbon atoms to form a helical tube. Carbon nanotubes typically have a diameter on the order of a fraction of a nanometer to a few hundred nanometers.
A carbon nanotube is known to be useful for providing electron emission in a vacuum device, such as a field emission display. The use of a carbon nanotube as an electron emitter has reduced the cost of vacuum devices, including the cost of a field emission display. The reduction in cost of the field emission display has been obtained with the carbon nanotube replacing other electron emitters (e.g., a Spindt tip), which generally have higher fabrication costs as compared to a carbon nanotube based electron emitter.
The manufacturing costs for vacuum devices (e.g., a field emission display) that use a carbon nanotube can be further reduced if the carbon nanotube is grown on the field emission substrate from a catalytic surface using chemical vapor deposition or other film deposition techniques. Nanotube growth can be conducted as a last deposition process preventing the degradation of the electron emitter properties by other device processing techniques or steps (e.g., wet processes).
Carbon nanotubes can also function as either a conductor, like metal, or a semiconductor, according to the rolled shape and the diameter of the helical tubes. With metallic-like nanotubes, it has been found that a one-dimensional carbon-based structure can conduct a current at room temperature with essentially no resistance. Further, electrons can be considered as moving freely through the structure, so that metallic-like nanotubes can be used as ideal interconnects. When semiconductor nanotubes are connected to two metal electrodes, the structure can function as a field effect transistor wherein the nanotubes can be switched from a conducting to an insulating state by applying a voltage to a gate electrode. Therefore, carbon nanotubes are potential building blocks for nanoelectronic devices because of their unique structural, physical, and chemical properties.
Existing methods for the production of nanotubes include arc-discharge and laser ablation techniques. Unfortunately, these methods typically yield bulk materials with tangled nanotubes. Recently, reported by J. Kong, A. M. Cassell, and H Dai, in Chem. Phys. Lett. 292, 567 (1998) and J. Hafner, M. Bronikowski, B. Azamian, P. Nikoleav, D. Colbert, K. Smith, and R. Smalley, in Chem. Phys Lett. 296, 195 (1998) was the formation of high quality individual single-walled carbon nanotubes (SWNTs) demonstrated via thermal chemical vapor deposition (CVD) approach, using Fe/Mo or Fe nanoparticles as a catalyst. The CVD process has allowed selective growth of individual SWNTs, and simplified the process for making SWNT based devices. However, the choice of catalyst materials that can be used to promote SWNT growth in a CVD process has typically been limited to Fe/Mo nanoparticles. Furthermore, the catalytic nanoparticles were usually derived by wet chemical routes, which are time consuming and difficult to use for patterning small features.
Another approach for fabricating nanotubes is to deposit metal films using ion beam sputtering to form catalytic nanoparticles. In an article by L. Delzeit, B. Chen, A. Cassell, R. Stevens, C. Nguyen and M. Meyyappan in Chem. Phys. Lett. 348, 368 (2002), CVD growth of SWNTs at temperatures of 900° C. and above was described using Fe or an Fe/Mo bi-layer thin film supported with a thin aluminum under layer. However, the required high growth temperature prevents integration of CNTs growth with other device fabrication processes.
Ni has been used as one of the catalytic materials for formation of SWNTs during laser ablation and arc discharge process as described by A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanet, J. E. Fischer, and R. E. Smalley in Science, 273, 483 (1996) and by D. S. Bethune, C. H. Kiang, M. S. de Vries, G. Gorman, R. Savory, J. Vazquez, and R. Beyers in Nature, 363, 605 (1993).
Regardless of how the nickel catalyst nanoparticles are formed, an oxidation layer forms on the catalyst nanoparticles in the ambient environment. Conventionally, hydrogen is used in the reduction phase of growth cycle, to remove the oxidation prior to growing the nanotubes. However, this must be done immediately prior to growing the nanotubes, and depending on the CNT growth technique and process conditions employed, such as thermal versus plasma enhanced, active gas composition, gas temperature, not all of the oxidation is removed. This results in a decrease of catalyst activity and in a reduction of the active site density leading consequently to the prevention of carbon nanotubes from growing on the catalyst as desired.
One known method for positioning carbon nanotubes comprises depositing the carbon nanotubes over a substrate having deep embossed holes, and shaking or vibrating the substrate until the carbon nanotubes have fallen into the holes. Another known method for positioning microtips comprises depositing a lift-off layer over a substrate with holes through the lift-off layer to the substrate. A material is deposited into the holes to form the tips with any of the material deposited onto the lift-off layer being removed when the lift-off layer is removed. Yet another known method comprises etching an emitter area into a dielectric beneath a gate layer and to a cathode layer on the substrate. A resistance layer is formed over the gate layer and on the surfaces within and around the emitter area. After etching in the emitter area to expose the cathode layer, a catalyst material is deposited on the cathode layer, followed by growth of nanotubes on the catalyst material.
Catalyst films are easily degraded by etch and clean chemistries which lead to reduced field emission and device performance characteristics. Any residual catalyst not removed leads to growth of emitters in undesired areas and leakage paths. One single nanotube or one cluster of nanotubes can render a display useless through the creation of “always on” emission sites. Such unwanted residue may also precipitate amorphous carbon during the nanotube growth process leading to leakage and shorting paths. Additionally, the presence of residue or veils (undesirable redeposition of material created by a dry etch process) from the resist operation can occlude carbon nanotube growth, thereby degrading the displays performance.
Accordingly, it is desirable to provide a process for producing a stable catalyst for carbon nanotube growth. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.